The present invention relates to engine brake control system and process for controlling a power unit of a vehicle adequately during an engine braking operation in which a driver decreases a throttle opening by releasing an accelerator pedal and thereby applies engine brake.
When the driver releases the accelerator pedal to apply the engine brake, some conventional systems are arranged to perform a fuel cutoff control for cutting off fuel supply to part or all of cylinders of the engine during such an engine braking operation, in order to improve the fuel economy.
In this type of the engine brake control system utilizing the fuel cutoff during the engine braking operation, an excess of engine braking effect might cause slipping or locking of the drive wheels of the vehicle on a low friction road or a curved road.
In this case, the vehicle might exhibit such unnatural behavior inconsonant with a driver's steering wheel operation, as to deteriorate the drive feeling and the stability of the vehicle.
Furthermore, in systems arranged to prolong the fuel cutoff period and improve the fuel efficiency by putting a hydrodynamic drive such as a torque converter in a lockup state of a direct mechanical coupling between input and output elements, the possibility of engine stall is undeniably associated with the wheel locking.
A conventional control system as disclosed in Japanese Patent Provisional Publication No. 3(1991)-194138 is arranged to weaken the engine brake to prevent unstable vehicle behavior by stopping the fuel cutoff stepwise in case of wheel slipping or locking.
Another conventional control system as disclosed in Japanese Patent Provisional Publication No. 5(1993)-155277 is arranged to predict engine stall by detecting an abrupt decrease of the engine speed, and change the torque converter from the lockup state to a converter state to prevent the engine stall.
However, the former conventional system must interrupt the fuel cutoff control to prevent wheel lock and unstable vehicle behavior at the cost of the advantage of the fuel cutoff control in the fuel economy.
The latter conventional technique must sacrifice the fuel advantage of the lockup by reducing the lockup time to prevent engine stall. Furthermore, the cancellation of lockup causes a reduction of the fuel cutoff time, resulting in further deterioration of the fuel consumption.